1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to the equipment that enables waveguide-mode multiplexing in optical communication systems.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A scalar waveguide mode is a guided electromagnetic wave having an electric- or magnetic-field distribution (hereafter referred to as optical-field distribution), in a plane perpendicular to the propagation direction, that is an eigenfunction of the scalar-mode equation for the waveguide. Scalar waveguide modes should be distinguished from vectorial waveguide modes that are eigenfunctions of the vectorial-mode equation for the waveguide. For a vectorial waveguide mode, if a loss or a gain of optical power in the waveguide (e.g., an optical fiber) is factored out, then the mode's optical-field distributions measured at two different locations along the waveguide will differ by a factor that reflects the overall phase change accrued by the mode between those two locations. Each waveguide mode (scalar or vectorial) is substantially a local eigenmode of the waveguide, and different scalar or vectorial waveguide modes are mutually orthogonal. Both scalar and vectorial waveguide modes form a respective full basis set and therefore, each scalar waveguide mode can be represented as a linear combination of vectorial waveguide modes, and vice versa. In general, an optical waveguide has a fixed number of waveguide modes whose optical-field distributions and propagation constants are determined by the waveguide structure, material properties, and optical frequency (wavelength).
At a transmitter of a waveguide-mode-multiplexing (WMM) communication system, a plurality of optical signals are independently modulated with data and coupled into a corresponding plurality of waveguide modes of an optical waveguide for transmission to a remote receiver. At the receiver, the received optical signals carried by the plurality of waveguide modes are separated from one another and demodulated to recover the data encoded onto the optical signals at the receiver. A waveguide-mode (WM) coupler is an optical device that can be used either at the transmitter for coupling the modulated optical signals into different waveguide modes of the transmission waveguide or at the receiver for separating from one another the optical signals carried by different waveguide modes of the transmission waveguide. Disadvantageously, prior-art WM couplers designed for selectively handling a relatively large number of waveguide modes suffer from relatively large optical insertion losses that increase rapidly with an increase in the number of optical channels.